1. Field of the Invention
The present invention relates to a liquid-sealing type vibration isolating apparatus which gives a vibration isolating effect on the basis of the flowing action of a fluid (liquid) sealed therein. More particularly, the present invention relates to a liquid-sealing type vibration isolating apparatus in which the liquid in a liquid chamber is vibrated at a certain frequency, the vibrating apparatus thereof having a simple structure, and isolation of vibration of a plurality of kinds is effectively accomplished over a wide scope ranging from low to high frequency regions.
2. Description of the Related Art
A vibration isolating apparatus, particularly an automotive engine mount must be capable of coping with a wide range of frequencies because the engine serving as a power source is used under various conditions ranging from idling operation to the maximum velocity of revolutions. For this purpose, there has already been invented an apparatus known as a liquid-sealing type engine mount (vibration isolating apparatus) in which two liquid chamber are provided and are connected with an orifice, such as the one disclosed in Japanese Unexamined Patent Publication No. 4-60231.
The aforesaid known apparatus is designed to have two orifices to cope with two kinds of input frequency within the low-frequency region. The apparatus can cope (vibration isolation) with two kinds of vibration such as engine idling vibration and engine shaking by operating these two orifices. These kinds of vibration have however a frequency with a range of from 10 to 30 Hz. An automobile engine is used under diverse and various circumstances, and the range of frequencies of vibration and noise propagating through the engine and the engine mount supporting the engine covers a wide region. Recently, in particular, vibration and noise associated with engine noise including a dull sound which is a vibration within the higher frequency region, in addition to the foregoing idling vibration and engine shake are forming an issue.
More recently, tuning of an engine mount is becoming more common with a view to isolating a dull sound associated with vibration of a relatively high frequency within a range of from 100 to 600 Hz. For the purpose of coping with a plurality of conditions as described above, there has already been known a liquid-sealing type vibration isolating apparatus having a liquid chamber and a fluid bag to change the volume at a specific frequency within the fluid bag, as disclosed, for example, in Japanese Patent Publication No. 6-29634.
In this known apparatus, the fluid bag is provided in the liquid chamber to change the volume thereof at a prescribed frequency, thereby causing a liquid in the liquid chamber on the vibration input side to flow via an orifice toward another side liquid chamber. More specifically, in the low-frequency region mainly comprising idling vibration, the liquid pressure in the liquid chamber on the vibration input side is increased so as to obtain a high damping property. In the high-frequency region, on the other hand, increase in the liquid pressure in the liquid chamber on the vibration input side is avoided to obtain a low dynamic spring constant. For recent automotive engine mounts, however, a vibration isolating apparatus should cover idling vibration against which resonance phenomenon should be avoided by reducing the dynamic constant as vibration within the low-frequency region, and vibration associated with engine shaking against which vibration should be inhibited by increasing the damping property.
Further, in this known apparatus, a fluid pressure generating device is need for causing the volume thereof to be changed, so that there are raised the following problems.
(1) An additional space has to be secured in an engine room. PA1 (2) The apparatus itself increases a production cost.
In order to achieve a vibration isolating apparatus capable of coping with these contradictory conditions, simple vibrating of the liquid in the liquid chamber on the vibration input side in the same or reversed phase is insufficient.
To cope with these multiple conditions, furthermore, there is already known an apparatus called a voice-coil type liquid-sealing type vibration isolating apparatus in which a liquid chamber is provided and which has a vibrator comprising a voice coil or the like vibrating at a certain frequency in the liquid chamber, as disclosed, for example, in Japanese Unexamined Patent Publication No. 5-149369. An apparatus of this type has however inevitably a complicated structure because of the necessity of a plurality of liquid chambers, a movable piece comprising a piston or the like in the liquid chamber and a voice coil for driving such a movable piece. The vibration isolating apparatus as a whole becomes unavoidably heavier because of shaking coils, permanent magnets and many other parts.